I've used Wikipedia for every single project I've ever done. I'm done my third year of university, I still use it, it's never steered my wrong.
Teachers hate it because it makes research stupid easy, not that they'd admit to that.
[Edited by SuperSkyline89, 5/14/2010 4:32:40 PM]
As Thrawn said, it's really a knee-jerk reaction to the whole "anyone can make changes" thing. They don't do any research into the matter, they just ban it.
It's not that hard to bypass the rules, though. All you have to do is not write "Wikipedia" as your source.